There are functionally at least two types of multiplex bus systems. One is a ring system in which various signal transmitters and signal receivers are connected to a single bus functionally arranged in the shape of ring. In succession each transmitter obtains a turn to transmit to a selected receiver. An example of such a system is U.S. Pat. No. 4,370,561 issued Jan. 25, 1983 to G. R. Briggs and assigned to the common assignee.
Another multiplex bus system is a spoke system in which a master control unit (MCU) is connected by a plurality of buses to a greater plurality of signal transmitters and receivers. In such a system, in turn each transmitter transmits its message to the MCU. Likewise the MCU transmits messages to the various receivers in turn.
In practice a plurality of receivers and transmitters are often combined in a common receiver-transmitter unit (RTU). In such a system the MCU sends a message previously received from a first RTU to the receiver of a second RTU and then receives a message from the second RTU's transmitter. In such systems, timing within the RTU is very important so that signals received by the RTU are properly interpreted within it and signals generated by the RTU are transmitted at the proper times to the MCU. Otherwise, erroneous actions with serious consequences could occur. As an example in a multiplex bus system used in an automobile, a signal transmitted by a horn switch connected to one RTU to the MCU and then to the horn by another RTU if subjected to improper timing could result in the horn not blowing and instead the head lights being turned off, an obviously intolerable situation.
In prior art systems complex and expensive timing circuits often employing crystal oscillators are employed in the RTU to insure that the timing system is held to close tolerances.